qseecom: use scm_call2 when bus_scaling is enabled

smcinvoke is not enabled on low-tier targets that need HLOS
to scale bus bandwidth explicitly, so it can still use
scm_call2 and then keep original bus scaling logic to avoid
bus scaling regression issue.

Change-Id: I3484109b58d1023e8732bf7f08a485b78f18ef65
Signed-off-by: Zhen Kong <zkong@codeaurora.org>